orbital architectures of sub-jovian EXTRASOLAR Planets

In the past two decades, the discovery and characterization of thousands of exoplanets has ranked among the most exciting developments in all of science. The recent triumph of the Kepler mission has not only shown that planet formation is relentlessly efficient within the Galaxy, but that the dominant mode of planet formation is one that produces so-called Super-Earths (often in multiples) with orbital periods less than ~100 days (Batalha et al. 2013). Normalized by the mass of the parent body, the orbital configurations of these systems often resemble the solar system’s giant planet satellites (Laughlin & Lissauer 2015). Nevertheless, there exists an important distinction: the preference for orbital resonances, that is strikingly clear within the solar system, is almost entirely absent in the extrasolar realm.

Theoretically, orbital resonances arise as a consequence of planet-disk interactions, and should be relatively common (see e.g. Cresswell & Nelson 2008). Therefore, their relative dearth among Super-Earths poses a critical challenge to planet formation theory. To address this discrepancy, two independent models have been proposed: Adams et al. (2008); Rein & Papaloizou (2009) suggested that in a sufficiently turbulent protoplanetary disk, resonances can be disrupted, while Goldreich & Schlichting (2014) argued that a specific choice of planet-disk interaction parameters can render resonances metastable within the nebulae. We have studied these ideas in quantitative detail, and showed that both of these theories are at odds with the observational data. Namely, Batygin & Adams (2017) showed that the turbulent resonance disruption process only operates for planets whose masses are considerably smaller than typical Super-Earths, while Deck & Batygin (2015) demonstrated that the Goldreich-Schlichting mechanism only affects systems with much more massive outer planets.

In light of the fact that neither of the previously proposed ideas successfully explain the largely non-resonant architectures of sub-jovian exoplanets, Batygin (2015) proposed that it is the resonance capture process itself that is inefficient. Specifically, we showed that probability of resonance capture is dramatically reduced in slightly (~2%) non-axisymmetric disks. Importantly, disk asymmetries of this magnitude (and greater) are not only an expected result of theoretical calculations, they are routinely invoked to explain observations of asymmetric glow of dust in young extrasolar systems (e.g. Mittal & Chiang 2015).